1. Field of the Invention
The present invention generally relates to a low-power, high-pressure discharge lamp, and in particular to a metal halide lamp having a discharge envelope vessel retaining a metal halide fill in a mercury atmosphere, and to a temperature control system for maintaining a stable lighting condition of the lamp, and maintaining a high luminous flux retention rate of the lamp.
2. Description of the Prior Art
Conventionally, a metal halide lamp has been fabricated under consideration of various quantitative restrictions such as restriction on lamp power consumption required for sufficient luminous energy or quantity of light in view of the provision of a lighting circuit, and in particular, when a lamp is used as a light source in an optical projector system, there have been required further restrictions such as a gap distance or an arc length between a pair of discharge electrodes. The electrodes, which are made of tungsten and the like material, are fabricated in a specific shape and size for increasing a luminance or brightness of an arc discharge portion to be produced between the electrodes in view of an optical requirement and an upper limit in quantity of a fill of mercury restricted for ensuring a pressure-proof property of an arc discharge tube.
Moreover, in recent years, there has been an increasingly strong demand for developing a metal halide lamp for use as a light source having characteristics of high luminance and high luminous flux retention rate in an essential part of an optical display incorporated in e.g. an optical projection system.
In particular, it is essentially important to optimize a contour of the discharge electrodes per se having a specific shape and dimension in fabricating a metal halide lamp because the design thereof exerts a great influence on the characteristics of the lamp such as a luminous flux retention rate, luminance of the arc discharge portion and lamp voltage varying rate.
However, in the conventional manufacturing method of the lamp, there has not been yet taught or established a guiding principle for providing a suitable design of electrodes to have optimum lamp characteristics, i.e., high luminous flux retention rate, high luminance of the arc discharge portion and small lamp voltage varying rate, under consideration of the restrictions of the lamp power, gap distance between electrodes, and upper limit of the fill of mercury. Therefore, the fabrication of an optimum metal halide lamp has been mainly carried out by reliance on experience.
In this conventional metal halide lamp, there have been drawbacks that, the discharge tube wall of quartz glass is easily reactive with a metal halide at a high temperature of about 1100.degree. C. or higher, and if the quantity of the metal halide sealed inside the tube is reduced by the reaction with the glass tube wall, the luminous flux retention rate is undesirably reduced to deteriorate the life property of the lamp.
Moreover, there have been problems that flickers and darkening phenomenon in the discharge tube wall may be easily caused undesirably due to scattering of the electrode evaporation to be adhered onto the inner face of the discharge tube during the light-on operation of the lamp, and also a color temperature change may be easily caused due to the change of the lamp voltage. The progressing degree of the blackening phenomenon is deeply related to the contour design of the electrodes.
When the heating of the discharge tube is excessively suppressed in temperature, there may be undesirably caused a lower-most part in temperature in the discharge tube wall behind the electrodes, which suppresses the evaporation of the metal halide in the discharge tube, resulting in deterioration of the luminous efficiency.
Thus, there has been an increasingly strong demand for establishing a reference guiding principle for providing a suitable design of discharge electrodes to have optimum lamp characteristics, i.e., high luminous flux retention rate, high luminance of the arc discharge portion and small lamp voltage varying rate in fabricating a metal halide lamp, under consideration of the restrictions of the lamp power, gap distance between the electrodes, and upper limit in mass of the fill of mercury.